Twist action friction drive

ABSTRACT

A twist action roller friction drive comprises a rotating drive bar which drives in rotation a roller the axis of rotation of which is inclined relative to the axis of a rotationally fixed driven member with which the roller engages. The inclined roller comprises a single annular roller urged from the inside into driving contact with the driven member by one or more hydrostatic pads. The driven member is a tube and the skewed annular roller is in frictional engagement with the bore of the tube. In a typical use, the tube is fixed to the carriage of a machine tool and is aligned with the machine axis. Oil for the hydrostatic pad(s) acting on the roller is supplied through the drive bar along the axis thereof At its trailing end, the drive bar rotationally drives a skewed roller assembly in which the annular roller is incorporated, the remote forward end of the drive bar being driven in rotation, as by an electric motor Axial movement of the driven member is principally determined by the angle of skew of the roller so that if this angle is made very small, similarly small precise axial movements of the driven member of as little as 1 nm (nanometre) or less can be readily achieved, per revolution of the drive bar. This permits a high speed drive motor and in turn velocity feed back control.

FIELD OF THE INVENTION

This invention relates to twist action friction drives.

PRIOR ART

From the prior art is known a twist action friction drive in which aroller means, driven in rotation about-: an axis inclined to that of adriven member, comprises three rollers all hydrostatically mounted aboutaxes inclined relative to that of the driven member. The arrangement iscomplex, and requires that all the rollers are mounted at precisely thesame inclination or helix angle if conflicting drive forces, leading toan unpredictable advance of the driven member, are to be avoided. Thisand other prior art arrangements are also disadvantageous in that therollers rotate around the outside of the driven member.

THE INVENTION

According to the invention, there is provided a twist roller frictiondrive comprising a rotating drive bar which drives in rotation rollermeans the axis of rotation of which is inclined relative to the axis ofa rotationally fixed driven member with which the roller means engages,wherein the inclined roller means comprises a single annular rollerurged from the inside into driving contact with the driven member by oneor more hydrostatic pads.

Preferably the driven member is a tube and the skewed annular roller isin frictional engagement with the bore of the tube. The tube ispreferably a steel tube, with a hardened and ground bore. In a typicaluse, the tube is fixed to the carriage of a machine tool, aligned withthe machine axis.

oil for the hydrostatic pads acting on the roller is preferably suppliedthrough the drive bar along the axis thereof. Such drive bar preferablycomprises a cylindrical tube.

At its trailing end, the drive bar rotationally drives a skewed rollerassembly in which the annular roller is incorporated, the remote forwardend of the drive bar being driven in rotation, as by an electric motor.The rotatlionally driven, forward end of the drive bar is preferablylocated, albeit indirectly, by hydrostatic bearings. The oil supply forthese bearings therefore constitutes a convenient source from which oilmay be supplied along the central axis of the drive bar to the skewedroller assembly, via suitable restictors.

At the skewed roller assembly, drillings are preferably provided tosupply oil to the bore and end faces of the roller, so that it is fullyfloating hydrostatically. Further drillings may supply oil to fourequianbyularly spaced hydrostatic pads which urge the skewed rollerassembly radially so that the roller is in frictional driving engagementwith the driven tube. Oil seeping from the roller assembly is preferablyreturned to a sump at the driven end of the drive bar, conveniently viathe interior of an extensible tube (which may be corrugated for thispurpose or of a telescoping design), which is secured between theforward end of the driven tube and a housing at the forward end of thedrive bar wherein drillings are provided to supply oil to thehydrostatic bearings which locate a rotating assembly carrying theforward end of the drive bar.

In the arrangement according to the invention, the axial movement of thedriven member is principally determined by the angle of skew of theroller. Thus, if this angle is made very small, small precise axialmovements of say 1 nm (nanometre) or less of the driven member canreadily be achieved per revolution of the drive bar, as compared, forexample, to axial movements of the order of 150 mm per revolution with acapstan drive, and 10 to 15 mm in the case of a hydrostatic nut drive.It follows that a relatively high speed drive motor can be employed,which is advantageous for velocity feedback control.

DESCRIPTION OF EMBODIMENT

A practical example of twist action friction drive is now described byway of example with reference to the accompanying drawings, in which:

FIG. 1 shows the complete drive in axial cross section;

FIGS. 2 and 3 are axial cross sections, in different planes, through theskewed roller assembly; and

FIG. 4 is a transverse cross section through the skewed roller assembly.

The twist action friction drive shown in FIG. 1 comprises, on the left,a driving head 10 wherein an assembly 12 within a housing 14 is able tobe driven in rotation by an electric motor 16 in order to impart arotational drive to the forward end of a drive bar 18 in the form of acylinder having a central axial passage 19. The construction of thedriving head 10 is not of importance in the present invention except forthe hydrostatic bearings 20 used in the rotating assembly 12. An oilreturn passage to the sump is indicated at 22. The motor 16 includes atachometer 17 for measuring the speed of the motor output and a controlunit 15 for controlling the motor speed in response to the output fromthe tachometer 17.

The invention is essentially concerned with the skewed roller assembly24 shown on the right-hand side of FIG. 1, and also in FIGS. 2 to 4. Thepurpose of this skewed roller assembly is to drive an outer tube 26,which in practice is fixed to the carriage of a machine tool, with aprecisely controlled linear, axial movement.

The skewed roller assembly is carried by a central shaft 28 fixed to andextending from the trailing end of the drive bar 18 with its axis at asmall angle of inclination to that of the drive bar. Fixed to the shaft28 to rotate concentrically therewith about the skewed axis is theannular roller assembly, formed as inner and outer parts 30, 32. Theskewed outer part 32 is urged by four hydrostatic pads 34 (see FIGS. 2and 4) to bear against the interior of the outer tube 26 at pointsaround a helical path, whereby the outer tube is frictionally drivenwith a linear axial movement as the roller assembly rotates.

FIG. 3 shows, in particular, that oil is also supplied, as indicated at36, to the axial face and the and faces of the roller assembly 30, 32,so that the assembly is fully floating, hydrostatically. Oil pressure atthe roller assembly, including that of the hydrostatic pads 34, iscontrolled by restrictors 38 in the central axial passage in the drivebar 18.

Oil seeping to the outside of the roller assembly 30, 32 and tending tofill the driven tube ahead of and to the rear of the roller assembly, isconveniently returned to the driving head 10, where the oil returnpassage 22 is located via the inside of a corrugated and axiallyextensible tube 40 fixed between the forward end of the driven tube 26and the housing of the driving head.

I claim:
 1. A twist action roller friction drive comprising a rotatabledrive bar, a driven member which is rotationally fixed relative to thedrive bar, and which is mounted for translational movement along a firstaxis, inclined roller means driven, in use, in rotation by the drive barabout a second axis which is inclined relative to the first axis, thedriven member being engaged by the roller means, wherein the inclinedroller means comprises a single annular roller and urging means forurging the annular roller into driving contact with the driven member,the urging means comprising at least one hydrostatic pad means situatedinside the annular roller.
 2. A friction drive as claimed in claim 1wherein the driven member is a tube and the annular roller frictionallyengages the internal bore of the tube.
 3. A friction drive as claimed inclaim 2 wherein the tube is of steel.
 4. A friction drive as claimed inclaim 3 wherein the inner surface of the tube is hardened and ground. 5.A friction drive as claimed in claim 1 wherein the drive includes a highspeed drive motor for rotating the drive bar, the motor having velocityfeedback control.
 6. A friction drive as claimed in claim 1 wherein thedrive bar is rotatable about an axis, and oil for the hydrostatic padmeans acting on the roller is supplied, in use, through the drive baralong the axis.
 7. A friction drive as claimed in claim 6 wherein thedrive bar comprises a cylindrical tube.
 8. A friction drive as claimedin claim 7 wherein the roller means further comprises a skewed rollerassembly in which the annular roller is incorporated, the skewed rollerassembly is situated at a first, trailing end of the drive bar and thedrive bar has a remote forward end which, in use, is driven in rotationby drive means and the forward end being mounted on hydrostaticbearings.
 9. A friction drive as claimed in claim 8 wherein the driveincludes a common oil supply for supplying oil to the hydrostatic padsmeans and to the hydrostatic bearing, the oil being supplied along thecentral axis of the drive bar to the skewed roller assembly.
 10. Afriction drive as claimed in claim 9 wherein the oil is supplied to theskewed roller assembly via a restrictor.
 11. A friction drive as claimedin claim 10 wherein, at the skewed roller assembly, drillings areprovided to supply oil to the bore and end faces of the roller, so thatthe roller assembly is fully floating hydrostatically on a spindleprojecting from the first bend of the drive bar.
 12. A friction drive asclaimed in claim 11 wherein further drillings for supplying oil to thehydrostatic pad means comprises four equi-angularly spaced hydrostaticpads, the roller assembly having the pads which urge the skewed rollerassembly radially so that the roller is in frictional driving engagementwith the driven tube.
 13. A friction drive as claimed in claim 12wherein oil seeping from the roller assembly is returned to a sump atthe driven end of the drive bar, via the interior of an extensible tube,which is secured between the forward end of the driven tube and ahousing at the forward end of the drive bar, wherein drillings areprovided to supply oil to the hydrostatic bearings which locate arotating assembly carrying the forward end of the drive bar.
 14. Afriction drive as claimed in claim 13 wherein the extensible tube iscorrugated or telescopic.
 15. A friction drive as claimed in claim 1wherein the extent of axial movement of the driven member, for a singlerevolution of the roller, is determined by the angle of skew of theroller.
 16. A friction drive as claimed in claim 15 wherein smallprecise axial movements of the driven member of the order of 1 nm(nanometre) are achieved per revolution of the drive bar.
 17. A machinetool having a carriage, a machine axis and a twist action rollerfriction drive as claimed in claim 1 wherein said driven member is fixedto or comprises an integral part of the carriage and is aligned with themachine axis.